Direct injection gasoline engines offer improved efficiency because fuel injected directly into a cylinder can reduce cylinder charge temperature. As a result, additional air may enter a cylinder as compared to an equivalent cylinder that has port injected fuel. Consequently, engine power and efficiency may be improved. In addition, direct injection gasoline engines may exhibit improved transient fuel control because there is less tendency for fuel to collect at a cylinder intake port of a direct injection engine than for a port fuel injection engine. However, direct injection engines may generate soot at higher engine speed and load conditions because there is less time available to atomize fuel in the cylinder. As a result, it may be useful to incorporate a particulate filter in the exhaust system of a direct injection engine. Gasoline engines include those engines fueled by pure gasoline or mixtures of gasoline and other fuels such as alcohols. Further, other fuels used in spark ignited engines are also included such as liquid propane gas (LPG) or compressed natural gas (CNG).
In U.S. Patent Application 2009/0193796 a system for treating exhaust gases of a gasoline engine is presented. In several embodiments, a three-way catalyst is followed by a particulate filter. The particulate filter may be coated with various combinations of platinum, palladium, and rhodium. The coated particulate filter may assist in the oxidation of soot that is held by the particulate filter. It may be beneficial to filter gasoline engine emissions with a particulate filter, but over time, a particulate filter may accumulate an amount of soot to the extent that it reduces engine efficiency by increasing backpressure in the exhaust system. The reference appears to provide little direction as to how to remove soot from a particulate filter. Therefore, the system described in the reference may cause engine performance to degrade over time. In addition, the three-way catalysts described in the reference operate at higher efficiencies when gases entering the three-way catalyst are near stoichiometric conditions. However, at stoichiometric conditions it may be difficult to regenerate a particulate filter. Conversely, the particulate filter may favor lean exhaust gases for regeneration, but conversion efficiency of NOx in a catalyst may degrade. The reference appears to offer little direction for overcoming these issues.
The inventors herein have developed a system for processing particulate matter of a gasoline engine, comprising: a gasoline engine having an exhaust system; a device including at least an oxidation catalyst and a particulate filter that is disposed in said exhaust system, said oxidation catalyst absent of an oxygen storage medium, said device an aftertreatment device located farthest upstream in said exhaust system; and a three-way catalyst disposed in said exhaust system downstream of said device.
Engine air-fuel control can be improved for an exhaust system that has a device including a particulate filter and a catalyst, the catalyst absent an oxygen storage media, by placing a three-way catalyst downstream of the device. For example, an oxygen sensor may be placed downstream of the device so that fuel delivered to the engine can be adjusted to provide a substantially stoichiometric mixture to the downstream catalyst to improve catalyst efficiency. In this way, it is possible to regenerate the particulate filter while continuing to convert engine exhaust gases from HC, NOX, CO to CO2 and H2O. In one embodiment, engine fuel can be adjusted in response to an exhaust gas oxygen concentration downstream of the particulate filter. By having a particulate filter that is absent of oxygen storage media, a downstream oxygen sensor may sense an oxygen concentration in the exhaust gases that better represents engine feed gas emissions when the particulate filter is not regenerating. Therefore, engine exhaust gases can be adjusted to increase conversion efficiency in a downstream three-way catalyst.
When the particulate filter is regenerating a portion of the oxygen in the exhaust stream may be used to oxidize soot held by the particulate filter. By sensing the oxygen concentration of exhaust gases downstream of the particulate filter, and upstream of the three-way catalyst, a control system can automatically account for oxygen consumed during particulate filter regeneration. Specifically, since the amount of oxygen in the exhaust gases downstream of the particulate filter reflects the amount of oxygen consumed by particulate filter during regeneration, the engine fuel calculations do not have to estimate the amount of consumed oxygen and engine fuel can be adjusted simply based on the oxygen concentration determined downstream of the particulate filter.
The present description may provide several advantages. In particular, the description provides a system that does not have to account for an amount of oxygen consumed during regeneration of a particulate filter. Therefore, a simplified system of higher reliability may be constructed. In addition, the present description provides for a operating an engine at substantially stoichiometric conditions while regenerating a particulate filter. Operating an engine near stoichiometric conditions during particulate filter regeneration may improve engine emissions since three-way catalysts convert exhaust gases efficiently near stoichiometric conditions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.